Stop logging `result` in `WebAsyncManager`

[sbrannen]

Overview

The changes made to WebAsyncManager in conjunction with #32340 resulted in the async result being logged at DEBUG level; whereas, the result was not logged prior to Spring Framework 6.1.5.

Since logging of the result is better left to applications -- for example, via a Filter that is conditionally applied for debugging purposes -- we should stop logging the async result in WebAsyncManager.

Related Issues

• Better protect against concurrent error handling for async requests #32340